1. Field of the Invention
This invention relates to a method for forming hypohalous acid. Such a method is particularly useful for preparing low chlorides hypochlorous acid.
2. Description of the Related Art
There are several methods of making hypochlorous acids. When the acids are subsequently to be used for reaction with organic compounds to make such compounds as chlorohydrins, it is generally desirable that the hypochlorous acids have a low content of chloride. Chloride ions are preferably in low concentrations because they contribute to the production of undesirable chlorinated organic byproducts such as di- and tri-chlorides and they accelerate decomposition of hypochlorous acid to chlorates.
Low-chloride aqueous solutions of hypochlorous acid (HOCl) are known to be made by spraying fine droplets of aqueous alkali metal hydroxides or alkaline earth metal hydroxides, for example sodium hydroxide, in a reactor dryer with chlorine gas to make hypochlorous acid gas and solid metal chloride. Creating the spray of fine droplets (50 to 200 microns in diameter) requires high pressures of up to about 1000 psig (about 6900 kPa) which requires a high energy input. The hypochlorous acid gas is condensed along with the water vapor to produce the desired aqueous solutions of hypochlorous acid. Such condensation in the reactor typically requires using refrigeration equipment in order to achieve necessary condensing temperatures of between about -5.degree. C. to 20.degree. C. The product is typically a concentrated hypochlorous acid solution of 35 to 60 percent by weight (wt %) hypochlorous acid. However, this process suffers from several disadvantages: First, there is difficulty in handling solid salt product which includes separation of solid salt particles from gas and removal from the reactor; second, the reactor must operate at high temperatures (e.g. 75-150.degree. C.) to vaporize all of the HOCl and water from the salt; third, the process requires high chlorine to alkali metal hydroxide molar ratios of typically greater than 22:1; and finally, the disclosed process is energy inefficient, requiring large temperature swings on the large recycle gas stream. For example, typically this gas is cooled from the reaction/drying temperature of 75-150.degree. C. to the HOCl/water condensation temperature of -5.degree. to 20.degree. C. and then reheated to 140.degree. C. for recycle to the reactor.
Another process for making aqueous hypochlorous acid is similar to the above described process in that a solution of alkali metal hydroxide is sprayed into a chlorine atmosphere resulting in HOCl vaporization and a dry solid salt. The primary difference with this second process is that the aqueous HOCl solution is produced by absorption of the HOCl in water as opposed to the condensation of the HOCl and water vapor. However, the difficulties of handling the solid salt, high chlorine ratios, and energy inefficiency are the same.
Another process uses an organic solvent to extract HOCl from a brine solution. This process suffers from a need to further remove the HOCl from the organic solvent to produce an aqueous HOCl solution, a need to remove residual solvent from the brine solution, and undesirable reactions of HOCl with the organic solvent.
Therefore, it would be desirable to have a process for preparing aqueous solutions of low chlorides hypochlorous acid which demonstrates any one of the following advantages over the above described methods: continuous and produces higher yields; operates at lower reaction temperatures than typical HOCl processes; does not require handling of solid salt by-products or a spray of fine particle size droplets of caustic; requires lower rates of chlorine gas input; requires less processing and less expensive equipment; and is more energy efficient (i.e., does not require large heating/cooling cycles or high pressure on the liquid feed).